$ \left(\dfrac{81}{4}\right)^{-\frac{1}{2}}$
Solution: $= \left(\dfrac{4}{81}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{4}{81}$ Figure out what goes in the blank: $\Big({\dfrac{2}{9}}\Big)^{2}=\dfrac{4}{81}$ So $\left(\dfrac{81}{4}\right)^{-\frac{1}{2}}=\left(\dfrac{4}{81}\right)^{\frac{1}{2}}=\dfrac{2}{9}$